Liquid crystal display (hereinafter, simply referred to as ‘LCD’) is a display device that displays a desired image by selectively enabling light emitted from a backlight unit to penetrate into respective pixels with the polarization of liquid crystals.
Among the liquid crystal displays, an in plane switching liquid crystal display (IPS-LCD) has advantages in that its view angle is wider than those of widely used twisted nematic (TN) LCDs. That is, the IPS-LCD has liquid crystals that are not vertically aligned but aligned in parallel with a transverse surface of the electrode by disposing on the same plane electrodes of cells in which liquid crystal are aligned. That is, when an electric filed is formed as shown in FIG. 1, the alignment direction of liquid crystals is changed on a screen of the liquid crystal display
FIG. 1 shows the movement of liquid crystal molecules in each liquid crystal pixel of widely used IPS-LCD. As shown in FIG. 1, when no electric filed is formed between a common electrode 10 and a pixel electrode 20, the liquid crystal molecules are disposed in each pixel so that they can be in parallel to a rubbing direction of the liquid crystal molecules. When the rubbing direction is parallel to the absorption axis of a polarizer adjacent to a backlight unit, this display device is called an O-mode IPS-LCD. Also, when the rubbing direction is vertical to the absorption axis of the polarizer adjacent to the backlight unit, this display device is called an E-mode IPS-LCD. An observer-side polarizer has an absorption axis vertical to the absorption axis of a polarizer adjacent to the backlight unit regardless of the modes. FIGS. 2 and 3 show the O-mode IPS-LCD. As shown in FIGS. 2 and 3, the IPS-LCD has polarizers formed both sides of liquid crystal cells so that their absorption axes can be vertical to each other. Since the light passed through a light source-side polarizer reaches an upper polarizer in IPS-LCD without causing any phase retardation when an electric filed is not formed in the polarizers, the light is not passed through the upper polarizer. As a result, pixels are in a dark state, generally called a normally black mode. As shown in FIGS., attention should be paid to the fact that the term ‘IPS-LCD’ used in the present invention includes Super-IPS, fringe field switching (FFS), reverse TN IPS, etc.
In the other case, when an electric filed is formed between the common electrode and the pixel electrode, an alignment direction of the liquid crystals is changed by the rotation of the liquid crystals. Therefore, the light that is passed through the light source-side polarizer is also passed through the observer-side polarizer, thus to allow pixels to emit light.
These liquid crystals in the IPS-LCD has more improved visibility than TN-LCD when the light is emitted aslant, that is, when the light is observed aslant by an observer. This is why the TN LCD has liquid crystal alignment layers disposed on and down the liquid crystal cells; and electrodes disposed on and down the liquid crystal cells. When the liquid crystals are aligned by an electric filed, a zone where the liquid crystals are aligned aslant to a vertical direction is formed, and thus the high phase retardation may be caused according to the alignment of the inclined liquid crystals. As a result, the light passed through the liquid crystals is not completely linearly polarized but elliptically polarized to cause light leakage where some light is leaked or not completely passed through the liquid crystals. When the light leakage occurs, the contrast ratio is deteriorated, which leads to the severely low visibility.
In the case of the IPS-LCD, a zone where the liquid crystals are aligned aslant to the vertical direction is not formed when the liquid crystals are re-aligned, and thus the changes in phase retardation according to the alignment of the inclined liquid crystals are not too high. Therefore, the IPS-LCD has been widely used as a liquid crystal display with a wide view angle.
However, a light path in the liquid crystal cells is lengthened in the case of the IPS-LCD when the light is emitted aslant, and the emitted light is more weakly elliptically polarized than when the light is emitted to the front side. Also, since the aslant emitted light is leaked on the polarizer, it is necessary to compensate for the phase retardation.
A view angle compensating film has been widely used to compensate for the phase retardation. Here, the view angle compensating film refers to a film having a phase retardation opposite to the phase retardation of the aslant emitted light so that its phase retardation can be induced inversely with respect to the phase retardation of the aslant emitted light.
However, since the phase retardation of the aslant emitted light is not uniform according to the emission angle or conditions of light, the kind or level of the phase retardation of a retardation film is generally determined by much minute research.
The retardation film functions to inversely compensate for the phase retardation which may be caused in liquid crystals or polarizers, and films having a phase retardation are generally used as the retardation film. The retardation film may be divided into an A plate, a C plate and a B plate, depending on the kinds of the retardation film.
Herein, the expression ‘A plate and C plate’ is meant to be divided according to refractive index anisotropy of the respective plates. Hereinafter, detailed descriptions of the A and B plates are as follows.
That is, materials through which the light is passed have refractive indexes (nx, ny, nz) with respect to x, y and z axes, respectively. Here, when a material has the same refractive indexes, this material is called isotropic, and when a material has partially or completely different refractive indexes, this material is called anisotropic. For convenience' sake, when it is assumed that a thickness direction of a film is z direction, one of two plane directions of the film is x direction, and the other of the plane directions is y direction, the refractive indexes are represented by refractive indexes in the directions as shown in FIG. 6, respectively.
Here, when a film has the same refractive indexes in two directions but different refractive indexes in one direction, this film is called a uniaxial film. Also, when a film has different refractive indexes in all three directions, this film is called a biaxial film.
Among the uniaxial films, when a film has different refractive indexes in a plane direction, this film is called an A plate. In this case, a refractive index of the A plate may be represented by the following Equation 1, and an in-plane phase retardation (Rin) in the A plate may be represented by the following Equation 2.nx≠ny=nz  Equation 1Rin=d×(nx−ny)  Equation 2
wherein, d represents a thickness of a plate (a film).
In Equation 1, when a film satisfies the requirements of Equation: nx>ny, this film is called a +A plate, and when a film satisfies the requirements of Equation: nx<ny, this film is called a −A plate.
Also among the uniaxial films, when a film has different refractive indexes in a thickness direction, this film is called a C plate. In this case, a refractive index of the C plate may be represented by the following Equation 3, and a thickness-direction phase retardation (Rth) in the C plate may be represented by the following Equation 4.nx=ny≠nz  Equation 3Rth=d×(nz−ny)  Equation 4
In Equation 2, when a film satisfies the requirements of Equation: nx<ny, this film is called a +C plate, and when a film satisfies the requirements of Equation: nx>ny, this film is called a −C plate.
Also, the biaxial film is referred to as a film whose in-plane and thickness-direction phase retardations are all varied. Among the biaxial films, a −B plate has a relationship of Equation: nx>ny>nz.
Since IPS liquid crystal cells themselves function as a +A-plate having a high phase retardation value, there has been proposed a technique using a −A plate as a view angle compensating film in order to compensate for the phase retardation in the prior art. The −A plate is manufactured by aligning discotic liquid crystals at constant alignment. Manufacturing the −A plate using the discotic liquid crystals as described above is difficult to be realized, and this manufacturing method has problems regarding the optical axis deviation, etc. Therefore, the −A plate has no sufficient performance. In consideration of the recent technologies, it is also impossible to manufacture a −A plate using the nematic liquid crystals.
As an alternative, there has been proposed a view angle compensating film that compensates for the phase retardation caused in IPS liquid crystal cells by stacking a +C plate on a −B plate, as shown in FIG. 4. However, this manufacturing method also has problems in that it is difficult for the view angle compensating film to secure a sufficient contrast ratio according to various view angles, and pixel colors may be generally tinged with red.
Furthermore, the conventional view angle compensating film was attached as a separate layer to a polarizer, and then used to compensate for the phase retardation caused in the IPS-LCD. However, this separate layer such as the view angle compensating film may cause an increase in the total thickness of a panel.